Method and device for setting the short circuit moment in electric motors, particularly in servomotors

ABSTRACT

A method and a device are proposed for setting the short circuit moment in electric motors, particularly in servomotors. For this purpose, the actual short circuit moment of the electric motor is measured and compared with a predetermined short circuit moment. Then the set short circuit current required to attain the desired moment is calculated. A predetermined value for the set short circuit current, which is a function of the calculated value, is then stored in an electronic control device for the electric motor. For this purpose, the device includes a memory in the control device for storing the predetermined set value. In this way it is possible in a simple and economical manner to equip even motors that are produced in manufacturing series and in which greater component tolerances exist, with a short circuit moment that lies within a narrow tolerance range.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 07/941,930 filed Sep. 8,1992, now U.S. Pat. No. 5,347,214.

This application claims the priority of Application Ser. No. P 41 29495.5-32, filed Sep. 5, 1991, in the Federal Republic of Germany, thesubject matter of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and to a device for settingthe short circuit moment in electric motors, particularly inservomotors.

In connection with electric motors, particularly servomotors, it isusually required that the tolerance range of the adjustment or motorforces lies within very narrow limits. The short circuit moment, thatis, the torque when the motor drive shaft is held immobile, isproportional to the actuation forces of the system equipped with themotor. However, in practice there exist a large number of powerinfluencing tolerances of the individual components of an electric motorso that a large margin of error results for the short circuit moment inthe conventional manufacturing and installation methods which isundesirable and also not permissible in many applications.

To obtain electric motors that have a narrower tolerance range, it hasbeen the practice in the past to subject the motors to an appropriatetest and to reject those displaying values outside of the tolerancerange. However, this is a very expensive method and, in addition, thetolerance range cannot be made arbitrarily small if costs are to be keptwithin an economically justifiable range. Another known possibility toobtain a narrow tolerance range for the short circuit moment is the useof very high quality and high precision components. However, this wayalso leads to high costs.

ADVANTAGES OF THE INVENTION

The method, according to the invention, of setting the short circuitmoment in electric motors, particularly servomotors, has the advantagethat a very narrow tolerance range can be realized in a simple mannerfor the short circuit moment, with it being possible to employeconomical, mass produced articles for the individual components of theelectric motor, that is, the servomotor. Compared to the prior art, thetolerance range can be narrowed considerably and the number of rejectscan be reduced. With the method according to the invention, noadditional individual components and no modifications to existingindividual components are required, but merely a modification oradaptation of a stored value. The device, according to the invention,for setting the short circuit moment in an electric motor, particularlya servomotor, exhibits the corresponding advantages and comprises anelectrical control device connected to the motor to control the currentthrough the motor, with the control device including, and beingcontrolled by, a memory storing a predetermined short circuit currentsetting that has been previously determined by measuring and/orcalculations for setting a desired short circuit moment in the motor.

Advantageous modifications and improvements of the method and the deviceaccording to the invention are possible.

The calculation of the short circuit current Iks to be set can be madein a simple manner according to the following equation:

    Mks=c*Iks-Mv

where Mks is the desired short circuit moment, c is a constant and Mv isthe loss moment of the electric motor. Advisably the actual shortcircuit current Iki is measured in this connection.

The maximum motor current can then be limited or regulated in a simplemanner as a function of the stored value for the set short circuitcurrent Iks, without it being necessary to make any structuralmodifications or additions.

As a feature of the apparatus according to the invention for setting theshort circuit moment in electric motors, the memory may be configured ina simple manner as a matchable or adjustable resistance arrangement.This may be realized by a potentiometer or a plurality of individualresistors. In the latter case, means for burning out such individualresistors are provided in a known manner for matching. To simplify andreduce the costs of manufacture and to realize the smallest possiblestructural height, the resistors may be configured as integratedresistors or film resistors.

However, the memory may also be configured as an electronic memory, forexample as an EPROM or EEPROM. The electronic storage of the memoryvalue is particularly simple and is suitable for mass production of suchelectric motors or servomotors.

Advisably, the memory is a component of a current limiting device in theelectronic control unit, with the short circuit current value Iks to beset stored in the memory determining the current limiting value. Inelectric motors equipped already with an electronic control deviceand/or with a current limiting device, the costs for setting the desiredshort circuit moment Mks are again noticeably reduced.

This applies correspondingly for the case where the memory is acomponent of a current regulating device in the electronic controldevice, in which case the value for the short circuit current to be setas stored in the memory determines the set value in the currentregulating device. Here again, very little cost is involved if a currentregulating device is already provided.

In order to realize a lower structural height, the electrical orelectronic control device may also be integrated within the electricmotor.

BRIEF DESCRIPTION OF THE DRAWING

One embodiment of the invention is illustrated in the drawing and willbe described in greater detail below. The sole figure is a block circuitdiagram for an apparatus according to the invention for setting theshort circuit moment in electric motors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the embodiment shown in the drawing figure, an electronic controldevice 10 is connected in series with an electrical servomotor 11, withthis series connection being charged with a direct supply voltage by wayof two supply terminals 12 and 13. However, the invention is not limitedto servomotors, other electric motors, for example, alternating currentor three-phase current motors may also be equipped with a device forsetting the short circuit moment. Additionally, the electronic controldevice 10 may also act in a different manner on servomotor 11 or on theelectric motor, for example, a parallel connection may also be provided.

The electronic control device 10 includes an electronic switchingelement 14 that is connected in series with servomotor 11 and isconfigured, for example, as a transistor, thyristor, triac or the like.This electronic switching element 14 is controlled by a current limitingdevice 15, with the current limiting value being predetermined by way ofa memory 16. Memory 16 may be configured as an electronic memory, forexample an EPROM or EEPROM, thus making storage of the current limitingvalue easy by way of purely a software program. Another possibility isto configure memory 16 as a resistance network in which case a pluralityof resistors connected in this network are configured as integratedresistors or film resistors. The storage of the current limiting valueis then effected in a known manner by burning out individual resistorsto obtain in this way the desired resistance value which, in turn,determines the current limiting value. A conventional solution is, forexample, the use of a potentiometer.

The purpose of the described arrangement is to provide a defined shortcircuit moment with a narrow tolerance range for servomotor 11. Sincethe short circuit moment is a function of a corresponding short circuitcurrent, the value for the corresponding short circuit current isdetermined and stored in memory 16. In this way, the short circuitmoment to be set is indirectly predetermined.

An alternative possibility is to provide, instead of the currentlimiting device 15, a current regulating device which likewise acts onelectric switching element 14. The short circuit current to be set isthen likewise provided as a set value by way of memory 16.

The method of setting the desired short circuit moment resides in thatinitially the actual short circuit moment Mki of servomotor 11 ismeasured without any influence from a current limiting or currentregulating device. The actual short circuit moment Mki is then thetorque exerted by servomotor 11 when the drive shaft is prevented fromrotating. The following relationship exists between the actual shortcircuit moment Mki and short circuit current Iki:

    Mki=c*Iki-Mv

where c is a constant and Mv is the loss moment of the motor which, in acertain configuration of an electric motor, is essentially always thesame. Correspondingly, the short circuit moment Mks to be set is alsorelated as follows to the short circuit current Iks to be set:

    Mks=c*Iks-Mv

Since c and Mv need to be determined only once and then always remainthe same, a certain short circuit current can always be associated witha desired short circuit moment to be set by way of this relationship. Inother words, the short circuit current Iks to be set can be calculatedby way of this equation. A corresponding value is then stored in memory16. If a comparison between the actual short circuit moment and the setshort circuit moment indicates that the deviation lies within thedesired tolerance range, it is not necessary to predetermine a shortcircuit current setting. Advisably the actual short circuit current ismeasured when the constants are calculated for the first time.

Particularly if memory 16 is configured as an electronic memory, theshort circuit current setting required for mass production can bepredetermined very quickly.

If a control device is already in existence, components that areprovided for servomotor 11, such as, for example, electronic switchingelement 14 or also a current limiting device or a current regulatingdevice may also be employed for the described electronic control device10. In many cases, the electronic control device 10 may also beintegrated in servomotor 11 in order to reduce its structural height andto obtain a compact unit.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

What is claimed is:
 1. A device for setting the short circuit moment inan electric motor comprising: an electrical control device connected tothe motor to control the current through the motor, and wherein thecontrol device includes, and is controlled by, a memory storing adesired short circuit current setting that has been previouslydetermined by at least one of measuring and calculations for setting adesired short circuit moment in the motor; and wherein the electricalcontrol device is integrated into the electric motor.
 2. A deviceaccording to claim 1, wherein the memory is a matchable or adjustableresistance arrangement.
 3. A device according to claim 2, wherein thememory is a potentiometer.
 4. A device according to claim 2, wherein thememory is composed of a plurality of individual resistors, with meansfor burning out such individual resistors being provided to permitmatching.
 5. A device according to claim 4, wherein the resistors areintegrated resistors or film resistors.
 6. A device according to claim1, wherein the memory is an electronic memory.
 7. A device according toclaim 1, wherein the memory is a component of a current limiting deviceincluded in the electrical control device, with the value for thedesired short circuit current stored in the memory predetermining thecurrent limiting value.
 8. A device according to claim 1, wherein thememory is a component of a current regulating device included in theelectrical control device, with the value for the desired short circuitcurrent stored in the memory predetermining the value to be set in thecurrent regulating device.
 9. A device according to claim 1, wherein thecontrol device further comprises a switching device connected to theelectric motor for controlling the motor current, and a regulatingcircuit connected to the switching device for controlling the switchingdevice to regulate the motor current as a function of the stored settingcorresponding to the set short circuit current.
 10. A device accordingto claim 9, wherein said switching device is a transistor having itscurrent carrying path connected in series with the motor and its controlelectrode connected to an output of the regulating circuit.
 11. A deviceaccording to claim 1, wherein the control device further comprises aswitching device connected to the electric motor for controlling themotor current, and a current limiting circuit connected to the switchingdevice for controlling the switching device to limit the motor currentas a function of the stored setting corresponding to the set shortcircuit current.
 12. A device according to claim 11, wherein saidswitching device is a transistor having its current carrying pathconnected in series with the motor and its control electrode connectedto an output of the limiting circuit.
 13. A device according to claim 1,wherein said electric motor is a servomotor.
 14. A device according toclaim 1, wherein said electric motor is an alternating current motor.15. A device according to claim 1, wherein said electric motor is athree-phase current motor.